You need Biochem to do well on the MCAT. If you take the class over the summer, you will not have time to adequately prepare for the MCAT and excel in Biochem. Also, if you take an end of summer or fall MCAT, it takes a month to get the score back, which will delay the time when your application will become complete and be transmitted to schools. Applying early in the cycle is recommended and starts in May or June, depending on which application service you will use.
I suggest you consider a gap year so you will have plenty of time to do well on every component of a good application, including buffing up your ECs, writing your Personal Statement, completing the Activities section, and not having to rush through your Secondary essays to get them in quickly.